1. Field of the Invention
The present invention relates to a satellite repeater for use with a satellite communication system and, more particularly, to a satellite repeater capable of concurrently transmitting (a) a plurality of fixed spot beams for communicating, for example, with a similar plurality of spaced-apart high traffic ground station areas, and (b) a scanning spot beam for sequentially communicating, for example, with a plurality of spaced-apart low traffic ground station areas on a time-division multiple access (TDMA) basis.
2. Description of the Prior Art
The current approaches to domestic-satellite systems divided along the lines of area-coverage and spot-beam concepts. Each system has its merits as well as disadvantages. A spot-beam satellite system allows high antenna gain and several reuses of the allocated frequency spectrum. In this regard see, for instance, U.S. Pat. No. 3,406,401 issued to L. C. Tillotson on Oct. 15, 1968. The disadvantage of such a system stems from the fact that each spot-beam covers only a small area. To avoid cochannel interference, a dead space between any two adjacent beams much larger than the beam coverage area (e.g., 3-dB contour) is required where the same frequency spectrum and polarization is used in adjacent beams. Also, there are regions needing service which do not have enough traffic to justify a dedicated spot-beam.
Area coverage satellites, such as used, for example, by AT&T/GTE, use broad antenna beams covering, for example, the whole United States. They are capable of providing service everywhere within the continental U.S.A. but lack channel capacity because the alloted spectrum can be reused at most once by polarization reuse. A more significant disadvantage, however, is the power penalty associated with the gain of an area-coverage antenna. The 3-dB contour gain of a U.S. coverage antenna is approximately 27-dB, and there appears little that can be done to improve it further. To obtain the same SNR as the previously mentioned spot-beam antenna system, the required RF power to transmit, for example, at a 600-Mb/s data rate would be 3 kW. Equivalently, one could use a 10 times larger diameter earth station antenna than used by a spot-beam system. Since neither alternative is practical, the link signal-to-noise ratio (SNR) must be compromised by approximately 10-dB. Even with a 10-dB sacrifice in margin, an additional 10-dB must be obtained through a combination of higher satellite transmitter power and larger earth station antennas.
Arrangements for using a movable beam in satellite, airborne, or mobile communication systems have also been disclosed. For example, U.S. Pat. No. 3,750,175 issued to R. M. Lockerd et al on July 31, 1973 discloses a modular electronics communication system comprising a plurality of radiating elements formed into an antenna array for transmitting and receiving communication frequency signals and employing a central processor to generate the transmitted signals and process the received frequencies through a manifold arrangement. Each radiating element connects to the manifold through a module made up of integrated microwave circuitry including a mixer coupled to a local oscillator and a phase shifter coupled to a beam steering computer. By means of the beam steering computer the antenna can be made to scan various preselected areas to primarily overcome tolerances in the satellite's or aircraft's attitude control system and maintain a beam at a desired target area. Additionally, the possible use of steerable beams and time-hopped steerable beams was suggested, but no implementation thereof shown, in Progress in Astronautics and Aeronautics, Vol. 33, pp. 503-531 at page 507 in the article "Characteristics and Applications of Multibeam Spacecraft Antennas", which was presented as Paper 72-530 at the AIAA 4th Communications Satellite Systems Conference, Washington, D.C., April 24-26, 1972.
In the more recently proposed Intelstat V satellite communication system, it has been proposed to concurrently use an overlapping fixed spot beam, a larger area zone coverage beam, and a still larger hemisphere coverage beam, each type of beam using a different frequency spectrum and/or polarization for transmitting its signals to avoid interference at receivers capable of receiving signals in more than one type of beam, for increasing satellite throughput capacity. Additionally, the concurrent use of both a plurality of fixed spot beams and an area coverage beam, which overlaps the spot beams and where all beams use the same frequency spectrum for transmission, has been proposed in the article "Spectral Reuse in 12 GHz Satellite Communication Systems" by D. O. Reudink et al in IEEE Conference Record ICC 77, June 12-15, 1977, Chicago, Ill., Vol. 3, at pp. 37.5-32 to 37.5-35. There, arrangements are disclosed for substantially cancelling the interference of the received signals originally transmitted in the area coverage beam with the desired spot beam signals received in each of the overlapped spot beam receiver areas.
The problem remaining in the prior art is to provide a satellite communication system which achieves an increase in capacity in a particular frequency spectrum over prior art multiple-beam communication systems while both avoiding interference between signals in beams used for both low traffic and high traffic ground station areas and reducing the power requirements by increasing the gain in transmissions to low traffic ground stations.